Battery suddenly draind down to zero.

I have XK 2007. I haven't had this problem, but on Jag forums I have seen this issue frequently, possibly because I followed the recommendations on the forum. Jag dealer service in a number of cases suggested that car was not being driven enough seems a crazy answer but if you understand the reason this problem occurs it makes more sense. 1. Apparently when you turn the car off, the computer systems still run for about 60 minutes. However if you lock the car after turning it off the computer runs for only 15 minutes and so battery drain is much less. Cars with a large amount of electronics/computers do continue to drain the battery. 2. If you don't run the car often enough, and many are only used infrequently, the battery does not get a chance to charge and it is recommended that you buy a trickle charger that is connected when the vehicle is not being used. I use a C-Tek 3300 and on my car there are holes for a 6mm bolt on the battery clamps to attach the ring connectors, maybe Jag realized you need to do this. The C-Tek is the model Jag uses with the Jaguar name on it and charges $150 but you can buy for $66. Get the extension chord BTW. The C-Tek will charge the battery until it's in a fully charged state and then trickle charge it after that. The unit doesn't get hot and the connectors are very well designed. I do this once a week and have had no issues with the battery so far. My buddy has a Porche and it has the same issue and if his car is not used for 5 days all the electronics turn off and you can only access the car with the old fashioned key. This problem is not limited to Jaguar. These cars constantly use power even when not being driven and it takes my C-Tek about 6 hours to get the battery charged to full each week.

Could be the alternator not charging the battery. I'd have the alternator tested.

Had exact same issue on 2010 - if you find an answer would love to know, if I get mine figured out will post back.